1. Field of the Invention
The present invention relates in general to magnetic field and current sensing, and more particularly to integrating a GPS receiver with a compassing sensor.
2. Description of Related Art
Magnetic field sensors have applications in magnetic compassing, ferrous metal detection, and current sensing. They may be used to detect variations in the magnetic field of machine components and in the earth's magnetic field, as well as to detect underground minerals, electrical devices, and power lines. For such applications, an anisotropic magneto-resistive (AMR) sensor, a giant magneto-resistive (GMR) sensor, a colossal magneto-resistive (CMR) sensor, a hall effect sensor, a fluxgate sensor, or a coil sensor that is able to detect small shifts in magnetic fields may be used.
Magneto-resistive sensors, for example, may be formed using typical integrated circuit fabrication techniques. Permalloy, a ferromagnetic alloy containing nickel and iron, is typically used as the magneto-resistive material. Often, the permalloy is arranged in thin strips of permalloy film. When a current is run through an individual strip, the magnetization direction of the strip may form an angle with the direction of current flow. As the magnetization direction of the strip changes relative to the current flow, its effective resistance also changes. Strip resistance reaches a maximum when the magnetization direction is parallel to the current flow, and reaches minimum when the magnetization direction is perpendicular to the current flow. Such changes in strip resistance result in a change in voltage drop across the strip when an electric current is run through it. This change in voltage drop can be measured and used as an indication of change in the magnetization direction of external magnetic fields acting on the strip.
To form the magnetic field sensing structure of a magneto-resistive sensor, several permalloy strips may be electrically connected together. The permalloy strips may be placed on the substrate of the magneto-resistive sensor as a continuous resistor in a “herringbone” pattern or as a linear strip of magneto-resistive material, with conductors across the strip at an angle of 45 degrees to the long axis of the strip. This latter configuration is known as “barber-pole biasing.” The positioning of conductors in a “barber-pole biasing” configuration may force the current in a strip to flow at a 45-degree angle to the long axis of the strip. These magneto-resistive sensing structure designs are discussed in U.S. Pat. No. 4,847,584, Jul. 11, 1989, to Bharat B. Pant, and assigned to the same assignee as the current application. U.S. Pat. No. 4,847,584 is hereby fully incorporated by reference. Additional patents and patent applications describing magnetic sensor technologies are set forth below, in conjunction with the discussion of FIG. 4.
Magnetic sensors often include a number of straps through which current may be run for controlling and adjusting sensing characteristics. For example, magnetic sensor designs often include set, reset, and offset straps. These straps can improve the performance and accuracy of magnetic sensors, but require driver circuitry for proper operation. Such circuitry has typically been located off-chip from the magnetic sensor, resulting in space inefficiencies. Similarly, other components, such as operational amplifiers, transistors, capacitors, etc., have typically been implemented on a separate chip from the magnetic sensor. Both signal conditioning and electrostatic discharge circuitry, for example, are typically located off-chip. Although such off-chip circuitry is adequate for many applications, for those where physical space is at a premium it would be desirable to have necessary circuitry integrated into a single-chip magnetic sensor, thereby conserving space.
One consequence of the space inefficiencies of multiple-chip magnetic sensors is the stunting of technological advances in the integration of compassing and positioning technologies. To take advantage of the functionality of both magnetic sensors and positioning technologies, at least one additional positioning chip is required. The Global Positioning System (GPS), the leading positioning technology, enables a GPS receiver to determine its position on the earth from a set of concurrently received signals transmitted by at least three of a constellation of GPS satellites. GPS receivers can also determine heading using the same signals used to determine position. However, in order to obtain an accurate heading, the GPS receiver must be moving at a speed of at least 10 mph. As a result, GPS has been successfully used for positioning in both handheld and vehicle-mounted systems, as well as for navigation in vehicle mounted systems (when traveling at a speed of at least 10 mph).
By combining the functionality of a magnetic field sensing device with that of a GPS receiver, a user can determine both direction (from the magnetic field sensing device) and position (from the GPS receiver), both when stationary and when moving. However, for handheld applications, such a combination may be unwieldy and inefficient due to the physical space requirements of a GPS receiver chip, a magnetic field sensing device chip, and a potential for additional chips required for magnetic field sensing device circuitry. Thus, a single-chip design that would minimize the physical space required to integrate a GPS receiver with a magnetic field sensing device would be desirable.